As said if the gauge is sitting correctly then the car should be running the correct temp.
My heater had always been very hot to the point that if you placed your hand over the vent you would need to move it after a few seconds as it was too hot for anything more.
When the temp gauge went out the heaters when on full where just tepid ,so married with the temp gauge problem in my case the stat was the answer to heat the whole car back up.
Sounds like it could be the bowden cables not opening /closing the flap correctly channeling the heat to the right place.
Hope it resolves it for you
@Jonno Bee